Exposure apparatuses are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that positions a reticle, an optical assembly having an optical axis, a wafer stage assembly that positions a semiconductor wafer, a measurement system, and a control system. Each stage assembly includes one or more movers, and each of the movers includes a coil array that interacts with a magnet assembly. The measurement system constantly monitors the position of the reticle and the wafer, and the control system controls each stage assembly to constantly adjust the position of the reticle and the wafer. The features of the images transferred from the reticle onto the wafer are extremely small. Accordingly, the precise positioning of the wafer and the reticle is critical to the manufacturing of high quality wafers.
In certain designs, the position of the respective stage measured by the measurement system is not an absolute position, but instead is relative to where the stage assembly happens to be when the measurement system is initialized. Therefore, it is imperative to determine an offset value that aligns the measured interferometer position to the actual poles of the magnet assembly for proper commutation of the movers. This offset value is often called the “commutation offset”.
The amount of force generated by each mover is a sinusoidal function of how accurately the commutation offset value is determined. For example, if the error between the calculated and real commutation offset is three percent, then the force generated by the mover is approximately ninety-eight percent of the maximum possible (and expected) force. However, for a commutation error of seven percent, the force falls to approximately ninety percent. Thus, it is important to accurately determine the commutation offset for each mover so that each mover operates efficiently and accurately.